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The endocannabinoid system (ECS) has been recently recognized as a prominent promoter of the emotional homeostasis, mediating the effects of different environmental signals including rewarding and stressing stimuli. The ECS modulates the rewarding effects of environmental stimuli, influencing synaptic transmission in the dopaminergic projections to the limbic system, and mediates the neurophysiological and behavioral consequences of stress. Notably, the individual psychosocial context is another key element modulating the activity of the ECS. Finally, inflammation represents an additional factor that could alter the cannabinoid signaling in the CNS inducing a “sickness behavior,” characterized by anxiety, anhedonia, and depressive symptoms. The complex influences of the ECS on both the environmental and internal stimuli processing, make the cannabinoid-based drugs an appealing option to treat different psychiatric conditions. Although ample experimental evidence shows beneficial effects of ECS modulation on mood, scarce clinical indication limits the use of cannabis-based treatments. To better define the possible clinical indications of cannabinoid-based drugs in psychiatry, a number of issues should be better addressed, including genetic variability and psychosocial factors possibly affecting the individual response. In particular, better knowledge of the multifaceted effects of cannabinoids could help to understand how to boost their therapeutic use in anxiety and depression treatment.

Endocannabinoid System

The endocannabinoid system (ECS) consists of endocannabinoids (eCBs), cannabinoid receptors (CBRs), biosynthesizing, and degrading enzymes. The main eCBs, anandamide (AEA) and 2-arachidonoyl-glycerol (2-AG), are degraded by distinct enzymes, the fatty-acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively. eCBs interact with a number of different molecular targets (Howlett et al., 2002) as the two CBRs (CB1R and CB2R) and with other G protein-coupled receptors (GPCRs) including GPR55 and GPR119, GPR18 which are expressed in different tissues, including the immune cells, and mediate a wide range of physiological functions (Chiang et al., 2015; Morales and Reggio, 2017). Furthermore, eCBs bind to other receptor subtypes, as the transient receptor potential vanilloid type 1 (TRPV1) cation channel (Tóth et al., 2009), the peroxisome proliferator-activated receptor, and glycine receptors (Zhang and Xiong, 2009). Finally, it has been demonstrated that CBRs can form heterodimers with other receptors including serotonin (5-HT), opioid and dopamine (DA) receptors (Farran, 2017).

The ECS have a prominent role in maintaining emotional homeostasis, mediating the effects of different environmental signals, including rewarding and stressing stimuli (Parsons and Hurd, 2015). Moreover, recent evidence showed that also the immune system interacts with the ECS. In particular, different inflammatory mediators alter CB signaling in the CNS (Figure 1).

ECS Control of Emotions and Behavior

The ability to integrate external and internal stimuli to select appropriate behaviors is crucial for individuals. The ECS regulates a number of physiological functions and mediates the crosstalk between different neurotransmitter systems, therefore representing a key player in the control of behavioral responses (Lutz et al., 2015). Indeed, a number of emotions and behaviors, such as fear, anxiety, depression, stress-coping and reward-driven behaviors are critically modulated by the ECS.

In particular, it has been demonstrated that CB1Rs are expressed in brain areas critically involved in the control of mood and behavior, including limbic and frontal regions, and regulate serotonin transmission (Herkenham et al., 1991; Nakazi et al., 2000). Preclinical and human studies evidenced that activity of the ECS regulates anxiety and depression and modulates behavioral responses to stress and reward (Table 1).

TABLE 1

Table 1. Experimental and human studies showing the role of the ECS in the regulation of mood and in mediating the responses to reward, stress and inflammation.

It has also been shown that the clinical effect of different antidepressants is mediated by a modulation of ECS activity in specific brain regions, including the hypothalamus, the amygdala and the hippocampus. In particular, chronic treatment with tricyclic antidepressants was associated with increased CB1R density in the hippocampus and hypothalamus and reduced hypothalamic-pituitary-adrenal (HPA) axis activation in response to stressing stimuli (Hill et al., 2006, 2009a).

Several genetic variants of the genes encoding the cannabis receptor type 1 (CNR1) and FAAH have been identified (Monteleone et al., 2010). CNR1 has been localized to chromosome 6q14-q15 (Hoehe et al., 1991; Zhang et al., 2004). It has been suggested that genetic variability affecting ECS functionality can influence the individual susceptibility to mood disorders (Monteleone et al., 2010). Accordingly, the AAT triplet repeat polymorphism of the CNR1 gene, and different single nucleotide polymorphism (SNPs) of CNR1 and FAAH genes have been associated with depressive symptoms and with major depression (Barrero et al., 2005; Domschke et al., 2008). In major depression, SNPs of the CNR1 may also influence the response to antidepressant treatment (Domschke et al., 2008; Mitjans et al., 2013). Moreover, it has been evidenced that CNR1 SNPs may interact with previous negative experiences increasing the susceptibility to depression (Juhasz et al., 2009). In particular, in patients with major depression, CNR1 SNPs may affect responsiveness of subcortical structures, including the amygdala and striatum, to social rewarding stimuli (Domschke et al., 2008).

Overall, data from animal models and clinical studies suggest that the ECS functionality could influence anxiety and depression. The mechanisms underlying the interaction between ECS and mood are different and include the modulation of activity in limbic areas involved in reward processing, regulation of other neurotransmitters (i.e., noradrenaline, NA; DA; 5-HT), and HPA activation in the stress response (Hill and Patel, 2013; Micale et al., 2013).

The ECS Mediates Reward

Mesocorticolimbic DA projections to the limbic system, including the amygdala, hippocampus and orbitofrontal cortex (OFC), are critically involved in mediating reward (Koob and Volkow, 2010). In addition to DA other neurotransmitter, including acetylcholine, opiate peptides, glutamate and GABA, are involved in reward.

ECS and Stress

Activation of the hypothalamic-pituitary-adrenal (HPA) axis represents the typical response to stress. While the HPA axis activity could be modulated by the ECS, conversely both stress and glucocorticoids could induce eCBs signaling in brain areas critically involved in behavioral responses, such as the hypothalamus and the amygdala (Hill and McEwen, 2010; Hill and Patel, 2013).

Experimental data suggest that the ECS could be crucially involved in mediating the neurophysiological and behavioral consequences of stress. In fact, stressful events may increase the synthesis of eCBs in the periaqueductal gray (Hohmann et al., 2005) and alter eCBs amount in brain areas associated with reward processing (Patel et al., 2005; Rademacher et al., 2008) thus modulating different responses, including stress-induced analgesia (Hohmann et al., 2005) and sensitivity to natural reward (Rademacher and Hillard, 2007). Notably, other neurotransmitter systems, including cholecystokinin signaling, are involved in the regulation of the behavioral consequences of stress (Kurrikoff et al., 2008).

It has been demonstrated that the behavioral effects of social stress in mice were associated with a selective alteration of the sensitivity of GABA synapses to CB1R activation. In particular, exposure to social stress in mice altered the CB1R-mediated control of GABAergic synaptic transmission in the striatum (Rossi et al., 2008). The specific role of stress is confirmed by the evidence that striatal synaptic alterations are prevented by glucocorticoid receptors blockade and reproduced by glucocorticoids administration. Finally, stress-induced synaptic alterations were rescued after either exposure to natural rewards (i.e., running wheel, sucrose) or administration of cocaine (Rossi et al., 2008). These data suggest that the ECS integrates different environmental stimuli, modulating their effects on striatal synaptic transmission. Notably, different types of rewards administered before the exposure to stress, may exert a protective effect against stress-induced synaptic alterations (De Chiara et al., 2010). In addition, human studies showed that ECS functionality could influence the response to stress (Neumeister et al., 2013; Pietrzak et al., 2014). In particular, different SNPs of the ECS have been associated to post-traumatic stress disorder (PTSD), influencing the effects of previous stressful events (Lu et al., 2008; Agrawal et al., 2012; Mota et al., 2015).

Experimental studies showed that in EAE specific proinflammatory cytokines, particularly TNF and IL-1β, are critically involved in the induction of mood alterations (Pollak et al., 2003; Gentile et al., 2015b). Notably, the synaptic alterations have been found in the striatum, a key structure involved in mood regulation both in rodents and humans (Mathew and Ho, 2006; Nestler and Carlezon, 2006; Kim et al., 2008; Zhang et al., 2008). In particular, striatal synaptic degeneration and dendritic spine loss have been found already during the early phases of EAE, independently of demyelination or clinical disability (Centonze et al., 2009). This data suggest that TNF-induced striatal synaptic dysfunction may be responsible of the behavioral manifestations. In line with these findings, intracerebroventricular injection of TNF in control mice was associated with anxious behavior (Haji et al., 2012). Conversely, intracerebroventricular administration of etanercept, a TNF inhibitor, reduced anxious behavior and prevented the synaptic alteration in EAE mice (Haji et al., 2012). In addition, elevated levels of IL-1β and IL-1β mRNA have been evidenced in the striatum of EAE mice, together with reduced dopaminergic transmission (Gentile et al., 2015b). Accordingly, blocking IL-1β signaling in EAE mice can reduce mood alterations and restore DA release in the striatum (Gentile et al., 2015b).

Inflammation and ECS

A complex bidirectional interaction exists between the ECS and the immune system. eCBs signaling has immunosuppressant actions, in particular, CB2R stimulation reduce both inflammation (Pandey et al., 2009) and circulating proinflammatory mediators (Croxford and Miller, 2003). Conversely, lipopolysaccharide administration modulates eCB signaling (Klein et al., 2003). In particular, microglial cells are recently emerging as key elements mediating the relationship between inflammation and the ECS. Microglia express CBRs, particularly CB2Rs, and is able to release both inflammatory mediators and eCBs (Stella, 2009; Boorman et al., 2016). Notably, CB2R stimulation in microglial cells promote the release of anti-inflammatory cytokines (Ma et al., 2015). Therefore, reduced CB2R stimulation may contribute to the pathogenesis of mood disorders. Accordingly, it has been evidenced that a polymorphism of the CB2R gene, producing reduced activation of the CB2R receptor, was associated with increased incidence of depression (Onaivi et al., 2008). Moreover, it has been demonstrated that treatment with both minocycline and antidepressant drugs was associated to reduced release of proinflammatory molecules by microglial cells (Burke et al., 2014; Su et al., 2015).

Several studies evidenced that the ECS plays a critical role in mediating the effects of inflammation on synaptic functioning and mood. Accordingly, in EAE mice, CB1Rs stimulation may counteract the TNF-mediated synaptic alterations (Rossi et al., 2011). In particular, administration of a CB1R agonist, prevented the alterations of striatal transmission produced by TNF on brain slices.

Interestingly, it has been shown that inflammatory signals and environmental stimuli interact with the ECS to produce mood changes. In particular, the effects of IL-1β administration on striatal transmission were similar to the alterations induced by social defeat stress (Rossi et al., 2008). Furthermore, administration of IL-1β receptor antagonist (IL-1ra) was able to revert the synaptic and behavioral effects induced by both IL-1β injection and social defeat stress. These results suggest that IL-1β is involved in inflammation-induced mood alterations and also play a key role in mediating responses to environmental stress (Koo and Duman, 2008; Goshen and Yirmiya, 2009; Norman et al., 2010).

Therapeutic Application of Cannabinoids

Cannabis contains numerous active components. In particular, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) represent the main phytocannabinoids (Huestis, 2007). In addition to smoked cannabis, different commercially available compounds have been produced (i.e., dronabinol, nabilone, Sativex®), differing in THC and CBD composition. Cannabis-derived treatments are currently approved for several clinical conditions, including spasticity, pain, nausea, and some epileptic conditions (Pacher and Kunos, 2013; Devinsky et al., 2014). However, despite numerous reports of beneficial effects on anxiety and depression, the clinical efficacy is still debated (Whiting et al., 2015; Turna et al., 2017). In particular, recent reviews and meta-analyses, concluded that evidence supporting clinical benefits of cannabis-based therapies in mood disorders is scarce, and limited to low-grade evidence supporting the beneficial effect of CBD in social anxiety (Bergamaschi et al., 2011) and of medical marijuana in PTSD (Walsh et al., 2017).

The limited approved clinical indications contrast with the ample evidence, derived by preclinical studies and also from anecdotal observations, showing beneficial effects of ECS modulation on mood. It should be considered that a number of factors may contribute to the lack of clear findings. It has been observed that most clinical studies were considered as at high risk of bias, generally for incomplete report of outcome measures and for inadequate blinding procedure (Whiting et al., 2015). In addition, clinical studies mainly explored the effects of synthetic principles, and studies exploring the effects of cannabis are limited. Furthermore, the role of genetic individual variability should be considered, as differences in CB1Rs sensitivity to phytocannabinoids may explain the ample variability of the effects reported by patients.

A specific confounding factor that limits the clinical evidences supporting the beneficial effects of cannabinoids on mood is the difficulty in controlling the placebo effect. It is worth noting that the placebo response could significantly influence the clinical effect of drugs acting on the ECS (Benedetti et al., 2011a; Di Marzo and Centonze, 2015). Placebo effect constitutes a complex phenomenon coming from both the social ritual represented by placebo administration and the psychosocial context (Benedetti et al., 2011a). Interestingly, it has been clearly demonstrated that both the placebo response and drugs activate the same biochemical pathways. In particular, the placebo analgesia is mediated by both the opioid system and the ECS (Wager et al., 2007; Benedetti et al., 2011b). Consequently, it has been demonstrated that the administration of rimonabant inhibits the non-opioid placebo response (Benedetti et al., 2011b). In clinical trials evaluating the efficacy of antidepressant medications, it has been shown that the placebo effect could be responsible for at least one-half of the beneficial effect (Kirsch and Sapirstein, 1998) representing a relevant confounding factor. Indeed, as ECS mediates both the antidepressant effect of drugs and the placebo response, patients responding to antidepressant drugs may also show a marked placebo response.

Finally, concerns about the risks associated to cannabis use have limited the clinical indications of medical marijuana. In particular, acute cannabis use has also been associated with worsening of anxiety (Crippa et al., 2009). Furthermore, it has been proposed that chronic consumption could be associated with increased risk of psychosis, cognitive impairment and addictiveness in predisposed subjects (Volkow et al., 2014; Ksir and Hart, 2016; Turna et al., 2017).

Conclusions

Emotional homeostasis is crucially modulated by the activity of the ECS. In particular, different environmental and endogenous stimuli could influence the emotional state by modulating the sensitivity to eCBs of different neurotransmitter pathways in multiple brain areas. Cannabis-based compounds could exert antidepressant effects through complex influences on different behavioral responses, such as those associated to reward, stress and inflammation, also depending on the individual psychosocial context. Overall, these issues make it difficult to demonstrate unequivocal relationships between ECS modulation and the effects on mood. Although targeting the ECS could represent a promising treatment option in different psychiatric conditions, future clinical trials should be designed to explore specific outcome measures, to reduce the individual variability and to consider the placebo response. In the last years, the ample diffusion of smoked or vaporized cannabis for recreational and therapeutic purposes has not been accompanied by measures aimed at promoting information about medical marijuana use. In this view, it is important to design specific interventions to overcome the gap between preclinical studies and clinical evidences on the potential therapeutic use of cannabinoids.

Author Contributions

MS and EI: work conception and design, drafting the work, work revision, final approval, and global agreement. LG: work conception and design, work revision, final approval, and global agreement. PM, ED, AB, and FB: work revision, final approval, and global agreement. DC: work conception and design, guarantor of integrity of entire study, manuscript revision for important intellectual content, final approval.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Panagis, G., Mackey, B., and Vlachou, S. (2014). Cannabinoid regulation of brain reward processing with an emphasis on the role of CB1 receptors: a step back into the future. Front. Psychiatry 5:92. doi: 10.3389/fpsyt.2014.00092